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Appendix B Common Questions
© National Instruments Corporation B-3 NI 6115/6120 User Manual
Analog Input and Output
Why is there a minimum sampling rate on the NI 6115?
The NI 6115 makes use of a pipelined ADC in order to achieve high
sampling rates. Sampling at rates below 20 kS/s can result in improper
digitization, which appear as noise in the acquired data.
How do I enable the programmable antialiasing filter on the
NI 6115/6120?
In LabVIEW, select Data Acquisition»Analog Input»Advanced Analog
Input»AI Parameter.vi from the function palette to set the filter values of
50 kHz and 500 kHz for the NI 6115, or to enable the 100 kHz filter for the
NI 6120, on a per channel basis. To disable the filter, set the filter value to
0.
Figure B-1. Setting Filter Values in LabVIEW
In NI-DAQ, use the AI_Change_Parameter function to set the filter
value. Set paramID to
ND_Digital_Filter. Set ParamValue to
ND_High for a filter value of 500 kHz on the NI 6115 or 100 kHz on the
NI 6120. Use
ND_Low for a filter value of 50 kHz on the NI 6115. Use
ND_None to disable the filter. The filter is disabled by default.
I have connected a differential input signal, but my readings are
random and drift rapidly. What is wrong?
Check your ground-reference connections. The signal may be referenced
to a level that is considered floating with reference to the device ground
reference. Even if you are in differential mode, the signal must still be
referenced to the same ground level as the device reference. There are
various methods of achieving this reference while maintaining a high
common-mode rejection ratio (CMRR). These methods are outlined in
Chapter 4, Connecting Signals.